Antioxidant-rich products promise an easy way to stave off disease. Simply swallow two softgels daily or knock back a glass of goji-pomegranate juice and the "supercritical" compounds will neutralize those nasty free radicals that threaten your health.

Such bold claims seem logical. There's evidence that free radicals, or oxidants, are involved in certain illnesses, including cancer and degenerative brain diseases.

And when oxidants turn up in our bodies — it happens when we turn food into energy or are exposed to infection, smoking and other triggers — we fight back by producing antioxidants that can soak them up like a sponge.

Thus a theory was born: Maybe oxidation and disease can be prevented by eating fortified foods or taking dietary supplements containing plant-based antioxidants, which include vitamins C and E, beta carotene and polyphenols (flavonoids).

But researchers now say antioxidants have been overhyped and widely misunderstood. Scientists haven't determined how antioxidants work in our bodies; it's also unclear whether dietary supplements have any beneficial effect. In some cases, studies suggest antioxidants may cause more harm than good.

One recent study found that antioxidant compounds caused fertility problems in mice. Though popular among athletes, antioxidants haven't been shown to improve performance or speed recovery. To the contrary, supplementing with antioxidants may blunt the beneficial effects of working out. And while some dietary antioxidants may have a role in cancer prevention, excessive doses of some vitamins can aggravate illness or even cause it, researchers say.

"People should be aware that there is little to no data supporting the use of antioxidants to protect against disease," said cardiologist Toren Finkel, chief of the Center for Molecular Medicine at the National Heart, Lung, and Blood Institute.

Yet "antioxidants" remains one of the hottest buzzwords in the health and wellness industry.

Manufacturers have emblazoned it on everything from water and cereal to alcoholic drinks. Last year hundreds of products with antioxidant claims were launched, and products containing the nutrients continue to be a strong area of development, said Carlotta Mast, editor in chief of newhope360.com, which tracks the market in natural, organic and healthy products.

In the U.S., sales of top antioxidant supplements hit $5 billion last year, up 2.3 percent over 2009, according to Nutrition Business Journal.

"Consumers have made an association between antioxidants and health," said Mast. "They have a general understanding that antioxidants help with free radicals, and they know free radicals are bad. So they see a functional beverage that's 'rich in antioxidants' and think, 'This will be healthy for me.'"

A natural byproduct of eating, drinking and breathing, free radicals are an unavoidable hazard of living.

"Oxygen oxidizes our food to produce energy, and the oxygen is reduced, mostly to water," said biochemist Barry Halliwell, a pioneering researcher in free radicals and disease. But some oxygen winds up as free radicals, unstable molecules that are missing an electron.

Desperate to regain its balance, a free radical will steal an electron from the nearest substance, whether it's cellular DNA, protein or fat. The theft alters the structure of the nearby victim, creating another unstable compound and triggering a chain reaction.

In response, our bodies naturally produce antioxidants that, like bodyguards, defuse free radicals by donating electrons while staying in balance themselves — a system people can strengthen through regular exercise.

But aging and exposure to environmental stressors from sunburn to pollution make it harder to keep up with antioxidant production, said Amy Howell, an associate research scientist at the Marucci Center for Blueberry and Cranberry Research at Rutgers University.

For example, X-rays create oxidative stress because "radiation splits the water to make free radicals," said Halliwell, a deputy president of the National University of Singapore. And "cigarette smoke is already full of free radicals that attack the lungs and other parts of the body."

Researchers have known for decades that diseases including heart disease, cancer, stroke and neurodegenerative disorders are linked to damage caused by free radicals. They also found that people who eat a diet rich in fruits and vegetables have lower rates of disease.

As a result, they hypothesized that taking antioxidants as supplements or fortified foods could decrease oxidative damage. But when antioxidant compounds were tested, the results were largely disappointing.

Beta carotene supplements didn't just fail to protect people against cancer, they increased the risk of lung cancer in smokers. Trials looking at cardiovascular disease, other cancers and strokes have been mixed, but most haven't found the hoped-for benefits. When Ironman triathletes supplemented with vitamin E for two months, it exacerbated oxidative stress and inflammation.

Meanwhile, free radicals aren't always bad. The oxidant hydrogen peroxide, for example, can help open blood vessels; removing it with antioxidant therapy can impair the body's ability to get oxygen to muscles.

There's also some evidence that what doesn't kill you can make you stronger: A little short-term free radical damage may activate pathways in the body that are protective in the long run, Finkel said.

"The real debate is whether we should let the radicals do their thing and not get in the way," said David Neiman, director of the Human Performance Lab at Appalachian State University. "Probably 90 percent of all people who exercise will do fine with a fruit- and vegetable-based diet. But those who engage in more stressful exercise — marathoners, ultrarunners and Ironman triathletes — may need extra help."

Elite marathon runner Wesley Korir, of Kenya, is one athlete who swears by antioxidant supplements, which he believes have kept him healthy, allowing him to do faster workouts and recover more quickly. Korir, winner of the last two Los Angeles Marathons, takes several supplements, including a liquid product with acai, a fruit that contains antioxidants.

"I used to get infections; I was lacking the nutrition factor," said Korir, who will compete in Sunday's Chicago Marathon.

Still, experts like Howell say individual supplements aren't the best way to get antioxidants. Once ingested, they can lose potency or be inadequately absorbed by cells. The type of antioxidant, the type of food a person eats and even individual gut bacteria can affect how the compound works in the body.

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